1. Field of the Invention
The present invention relates to data storage devices of the type that write and/or read data on the surface of magnetic, optical, or magneto-optical storage media. More particularly, the invention relates to a system and method for managing defects within the servo regions of data storage media adapted for use in such apparatus.
2. Description of the Prior Art
By way of background, data storage apparatus such as disk and tape drives have transducers (also known as “heads”) that read and write data on the surfaces of data storage media that move relative to the transducers in either a rotational mode (disk drives) or a streaming mode (tape drives). The data storage media may be fixed or removable, and they can be adapted for magnetic, optical or magneto-optical data storage.
Servo information is used to maintain highly accurate positional relationships between the transducers and their associated data storage media during data read/write operations. In tape drives, the servo information is commonly recorded in servo sectors that are interspersed with the data regions that store user data on each longitudinal track of the tape media. In analogous fashion, the servo information of disk drives is commonly recorded in servo sectors that are interspersed with the data regions that store user data on each concentric track of the disk media. This is sometimes referred to as sector servo recording. Because the servo sectors are generally placed at the same circumferential locations on each track, they are aligned in servo regions that extend in a cross-track direction. Typically, there are multiple (e.g. 80-90) servo regions per disk.
The servo information on disk and tape media is sometimes prone to error as a result of media defects, recording defects, or a combination of both. Such errors can have a significant effect on storage device performance. For example, if a disk drive transducer is incorrectly positioned while in a track following servo mode due to a servo error, it could write data on an adjacent track and thereby obliterate previously recorded data. Such data losses are generally non-recoverable.
Presently, data storage devices use estimated transducer position as the main mechanism for identifying a defective servo condition. If the transducer position determined from reading servo information in a servo sector does not fall within a predetermined range of the position estimated from prior servo information, the determined transducer position is assumed to be in error. A deficiency of this method is that it is not reliable when the error produced by a servo defect is such that the error falls within the range of the estimated position.
There are methods for providing a storage device with predetermined defect information that accounts for servo sector defects. One method is to detect defective servo sectors during manufacturing and then mark one or more nearby data sectors as unusable. For example, data sectors that immediately precede and follow the defective servo sector on the same track can be marked. In some cases, data sectors on adjacent tracks are marked to provide further protection. Another method of handling servo defects is to apply compensation signals that attempt to correct the defective servo information. In particular, compensation is provided for the servo sector PES (Position Error Signal) bursts used to maintain a transducer over its associated track centerline while in a track-following mode. Both of the foregoing methods have significant disadvantages. For example, marking data regions as unusable reduces available storage media real estate. Compensating for defective servo information assumes that a transducer is already located above a track centerline. If the transducer is not so positioned, the compensation information will be inaccurate. Moreover, the determination of servo compensation information requires sophisticated equipment and is a task reserved for the device manufacturer at drive manufacture time. There is no ability to add to the compensation information should additional servo defects occur in the field.
Methods exist for mapping defects occurring in storage media data regions. Indeed, this is commonly done in so-called “No-ID” data sector disk drives during the conversion of logical addresses specified by a host device to the physical addresses arranged on the media. According to the usual technique, a data sector defect map stored in RAM (Random Access Memory) is used to skip over defective data sectors as the drive formatter electronics perform the address conversions. Although ideal for mapping data defects, this method is not suitable for mapping servo defects. Data is accessed along tracks and conventional data defect mapping techniques are designed to be efficient for locating defects in this manner. Unlike data defects, servo defects do not have precise track locations. As previously stated, servo sectors are adjacently arranged in a cross-track direction. Because track-to-track spacing is generally much less than the spacing between successive servo sectors along a data track, defects that affect a given servo sector are much more likely to affect servo sectors in adjacent tracks than they are to affect servo sectors on the same track.
An alternative servo defect handling method is therefore needed. What would be particularly desirable is a servo defect management scheme wherein servo defects can be identified in advance and used during drive operations to avoid transducer positioning errors. Preferably, the servo defect management scheme will be optimized to make efficient use of drive processing and memory resources. The ability to modify the servo defect information should additional servo defects arise following drive manufacture would also be desirable.